The present invention relates to a porous substance useful as gas-absorbing materials or catalyst supports, and a process for producing such a porous substance.
Porous substances including metal oxides such as zeolite, silica and alumina and activated carbon are industrially valuable as catalysts, catalyst supports, adsorbents or the like. In particular, it has been known that substances with highly developed pores are high in reactivity and absorption selectivity and in dispersibility of catalytic activated species. In view of this, carbon nanotubes, carbon nanohorns, and boron nitride nanotubes have been drawing an attention. However, since the large-scale production of these substances are difficult, there is a limit in industrial views to produce these substances at a low production cost.
Further, important future roles of porous substances are applications thereof to storage and transport of hydrogen or methane. In particular, storage and transport of hydrogen are technically difficult but are significantly important.
Hydrogen has been used in various industrial fields such as chemical industries and recently expected to be an energy for the future. As the result, studies mainly regarding fuel cells have been progressed. However, since hydrogen gas is large in volume per calorie and requires a large amount of energy for liquefaction, there is a problem that it is difficult to store or transport hydrogen gas as it is (see, for example, non-patent document 1 below). Therefore, a method has been demanded in which hydrogen is efficiently transported and stored for the case where a fuel cell is used in transportation devices such as fuel cell-powered vehicles or as decentralized electric sources.
As such a method, there has been proposed a method wherein hydrogen is stored and transported in the form of liquid hydrogen. However, there are problems that liquid hydrogen is difficult in handling because its liquefying temperature is −253° C. which is extremely low and the energy required for liquefaction of hydrogen is enormous, leading to a low overall energy efficiency (see, for example, non-patent document 2 below).
On the other hand, a method has been put in practical use, wherein hydrogen is transported and used as a high pressure gas. However, this method has problems that such a high pressure gas is dangerous in handling and it is difficult to compress hydrogen gas to a small volume even with an extremely high pressure of 35 MPa because the resulting volume is still large (see, for example, non-patent document 2 below).
Absorption of hydrogen to a hydrogen-absorbing alloy is also an effective method. However, the alloy can absorb hydrogen in an amount of usually on the order of 3 percent, which is insufficient to be used in transportation devices, and makes the weight thereof too heavy. Further, the alloy has disadvantages that it requires lots of energy to discharge hydrogen, resulting in poor energy efficiency and also makes the system more complicated (see, for example, non-patent document 4 below).
Alternatively, it has been considered to use a hydrogen-absorbing material as a method for compacting hydrogen gas for transportation purposes. This method has features that it is capable of discharging hydrogen at ambient temperatures and thus simplifying the system and is high in energy efficiency because of no necessity of heat to discharge hydrogen. Therefore, the development of materials for use in this method have been rigorously carried out. It has been reported that materials such as carbon nanotubes and carbon nanofibers exhibit high absorbability (see, for example, non-patent document 5 below). However, reproducibility of these materials are in question. Under the current situations, it is hardly say that the development of a hydrogen-absorbing material with sufficient reproducibility and high absorbability has been accomplished.
Therefore, the development of a material with high absorbability has been demanded, and a consideration has been given to the use of materials with pores, the size of which is in the same level as hydrogen, as such high absorbability materials. Examples of the materials include the above-mentioned carbon nanotubes and carbon nanofibers. In addition, the use of various materials mainly such as carbonaceous material have been attempted. Further, there are some reports introducing boron nitride nanotubes or porous complexes as materials other than carbons (see, for example, non-patent documents 6 and 7 below). However, there are some materials exhibiting high absorbability, in these reports, but the matter of fact is that the data therein are not reliable.
(1) Non-patent document 1: an article written by Kobayashi in “Kihou Energy Sogokogaku” vol. 25, No. 4, pages 73-87 published by The Institute of Applied Energy in 2003
(2) Non-patent document 2: an article written by Kuriyama in “Energy and Resources” vol. 24, No. 6, pages 23-27 published by Japan Society of Energy and Resources in 2003
(3) Non-patent document 3: an article written by Akiyama et al. in “Engine Technology” vol. 5, No. 3, pages 43-47 co-edited by Society of Automotive Engineers of Japan, Inc and The Japan Society of Mechanical Engineers in 2003
(4) Non-patent document 4: an article written by Akiba in “Engine Technology” vol. 5, No. 3, pages 36-42 co-edited by Automotive Engineers of Japan, Inc and The Japan Society of Mechanical Engineers in 2003
(5) Non-patent document 5: an article written by A. Chambers et al. in “The Journal of Physical Chemistry B” (U.S.A), vol. 102, pages 4253-4256, published by The American Chemical Society in 1998
(6) Non-patent document 6: an article written by Renzhi Ma et al. in “Journal of the American Chemical Society” (U.S.A), vol. 124, pages 7672-7673, published by The American Chemical Society in 2002
(7) Non-patent document 7: an article written by Nathaniel L. Rosi et al. in “Science” (U.S.A), vol. 300, pages 1127-1129, published by American Association for the Advancement of Science.